Well control is an important aspect of oil and gas exploration. When drilling a well, for example, in oil and gas exploration applications, safety devices must be put in place to prevent injury to personnel and damage to equipment resulting from unexpected events associated with the drilling activities.
Drilling wells in oil and gas exploration involves penetrating a variety of subsurface geologic structures, or “layers.” Occasionally, a wellbore will penetrate a layer having a formation pressure substantially higher than the pressure maintained in the wellbore. When this occurs, the well is said to have “taken a kick.” The pressure increase associated with the kick is generally produced by an influx of formation fluids (which may be a liquid, a gas, or a combination thereof) into the wellbore. The relatively high pressure kick tends to propagate from a point of entry in the wellbore uphole (from a high pressure region to a low pressure region). If the kick is allowed to reach the surface, drilling fluid, well tools, and other drilling structures may be blown out of the wellbore. These “blowouts” may result in catastrophic destruction of the drilling equipment (including, for example, the drilling rig) and substantial injury or death of rig personnel.
Because of the risk of blowouts, blowout preventers (“BOPs”) are typically installed at the surface or on the sea floor in deep water drilling arrangements to effectively seal a wellbore until active measures can be taken to control the kick. BOPs may be activated so that kicks are adequately controlled and “circulated out” of the system. There are several types of BOPs, one common type of which is an annular blowout preventer.
Annular BOPs typically comprise annular, elastomeric “packing units” that may be activated to encapsulate drillpipe and well tools to completely seal about a wellbore. In situations where no drillpipe or well tools are within the bore of the packing unit, the packing unit can be compressed to such an extent that the bore is entirely closed, acting as a valve on the wellbore. Typically, packing units are used in the case of sealing about a drillpipe, in which the packing unit can be quickly compressed, either manually or by machine, to affect a seal about the pipe to prevent a well from blowing out.
An example of an annular BOP having a packing unit is disclosed in U.S. Pat. No. 2,609,836, (“Knox”) which is incorporated herein by reference in its entirety. The packing unit includes a plurality of metal inserts embedded in an elastomeric body. The metal inserts are typically spaced equal radial distances from one another about a longitudinal axis of the packing unit. The inserts provide structural support for the elastomeric body when the packing unit is radially compressed to seal against the well pressure. Upon compression of the packing unit about a drillpipe, or upon itself, to seal against the wellbore pressure, the elastomeric body is squeezed radially inward, causing the metal inserts to move radially inward as well.
FIG. 1 is an example of a background art annular BOP 101 including a housing 102. The annular BOP 101 has a bore 120 extending therethrough and is disposed about a longitudinal axis 103. A packing unit 105 is disposed within the annular BOP 101 about the longitudinal axis 103. The packing unit 105 includes an elastomeric annular body 107. The packing unit 105 includes a bore 111 concentric with the bore 120 of the BOP 101.
The annular BOP 101 is actuated by fluid pumped into opening 113 of a piston chamber 112. The fluid applies pressure to a piston 117, which moves the piston 117 upward. As the piston 117 moves upward, the piston 117 translates force to the packing unit 105 through a wedge face 118. The force translated to the packing unit 105 from the wedge face 118 is directed upward toward a removable head 119 of the annular BOP 101, and inward toward the longitudinal axis 103 of the annular BOP 101. Because the packing unit 105 is retained against the removable head 119 of the annular BOP 101, the packing unit 105 does not displace upward from the force translated to the packing unit 105 from the piston 117. However, the packing unit 105 does displace inward from the translated force, which compresses the packing unit 105 toward the longitudinal axis 103 of the annular BOP 101. In the event a drill pipe 130 is located along the longitudinal axis 103, with sufficient radial compression, the packing unit 105 will seal about the drill pipe into a “closed position.” The open position is shown in FIG. 2 while the closed position is shown in FIG. 3. In the event a drill pipe is not present, the packing unit 105, with sufficient radial compression, will completely seal the bore 111.
An example of the packing unit 105 used in an annular BOP 101 is shown in FIG. 4. As before, the packing unit 105 includes an elastomeric annular body 107 and may include a plurality of metallic inserts 109. The metallic inserts 109 may be distributed at equal radial distances from each other in the elastomeric annular body 107 of the packing unit 105. The packing unit 105 has a bore 111 therethrough.
Elastomers of higher modulus have been used to provide annular packers having longer fatigue life. However, high modulus elastomers require higher closing pressure and more flexing cycles to seal the pipe or open hole. A packer with extreme stiffness may not close at all.
When incorporating fibers within a rubber compound, it is common to orient the fibers in the mill/extrusion grain direction. In comparison to unreinforced rubber compounds, fiber reinforced rubber compounds have increased modulus and decreased elongation along the mill grain. The unidirectional orientation of elastomer macromolecular chains and filler particles occurring in processes such as milling, extrusion or calendaring is called “grain”, which results in anisotropy of elastomer vulcanizates. Short fibers mixed in elastomer compounds are longitudinally oriented along the grain direction, which provide the anisotropic properties that are usually much more significant than those provided from unreinforced rubber compounds. The modulus increases and elongation decreases across the grain direction as well but is much slighter than those properties with the grain direction. In other words, a fiber reinforced rubber slab is physically anisotropic, that is, the modulus with the grain orientation can be significantly higher than that across the grain or that of an unreinforced control. However, the elongation of fiber reinforced rubber compounds with the grain will decrease less than the increase of the modulus.
The traditional packing units for BOPs use nitrile rubber (NBR) for the elastomeric annular body NBR, which provide good physical properties and oil resistance.
Complicated annular BOP operations require improved packers. Accordingly, it would be desirable to provide improved packers for annular BOP operations.